Among the liver’s many functions are: filtration of blood, metabolism of drugs and toxins, production of bile, a critical component of fat digestion, management of cholesterol and storage of a form of sugar known as glycogen for use when sugar is required but food is not available. To keep up with all these critical functions, the liver has an enormous blood supply. In fact, the liver receives the highest percentage of the blood pumped by the heart.
The flow of blood into the liver comes from two sources. Once the lungs load the blood with oxygen, the heart pumps the blood through the body’s main blood vessel, the aorta, and then on over to the liver. This is how blood is supplied to most organs in the body. But because the liver is unique and has so many vital roles to play, it has a second source of blood. This blood comes to the liver via the digestive system, after it has delivered oxygen there and picked up nutrients, like cholesterol and sugar, and other substances, such as any oral medications or toxins consumed by the pet. These nutrients and substances are then brought to the liver by this additional blood supply, where they are metabolized and, in the case of toxins, detoxified. This blood then returns to the heart and lungs where it is re-oxygenated and the process is repeated.
Before birth, blood bypasses the developing liver and passes through the umbilical cord to the placenta where the mother’s body supplies the necessary functions not yet provided by the puppy or kitten’s developing liver. Shortly after birth, the vessel bypassing the liver in the ‘infant’ closes, and blood is circulated through the newborn’s liver. If the vessel does not close, blood continues to shunt around the liver and a condition known as a congenital portosystemic shunt develops. A congenital portosystemic shunt is a form of birth defect. Older pets can also have abnormal shunting vessels in their livers called acquired portosystemic shunts. This condition usually is a result of long-standing liver disease, such as cirrhosis.
Because the liver filters blood, manages cholesterol, stores sugar and iron and aids in digestion, pets with a liver shunt can exhibit a range of clinical signs associated with abnormalities of these critical functions. Many puppies and kittens with a liver shunt are runty or smaller than the others in the litter because the nutrient-rich blood from the digestive tract bypasses the liver. For the same reason, their liver is also smaller. Anemia is common, in part due to abnormal iron metabolism. A certain type of urine crystal, which can turn into bladder stones, can develop in dogs with liver shunts. Pets with bladder stones may urinate frequently or have blood in the urine. Because a normal liver metabolizes any toxins and drugs in the bloodstream, pets with liver shunts may be more sensitive to the effects of certain medications.
Liver shunts in both dogs and cats may manifest as behavioral or neurologic abnormalities because the liver does not properly remove products of digestion that can affect the brain. Drooling is common in cats and dogs and cats may have seizures, act blind or exhibit bizarre behavior, especially after eating. Nutrient-rich blood from the digestive tract bypasses the liver via the shunting blood vessel and heads to the brain, where these substances can cause neurological disturbances. Anti-seizure medication is often part of the treatment protocol for pets with liver shunts.
Although any dog or cat can develop a portosystemic shunt, there are some breeds that may be at greater risk, including Miniature Schnauzers, Yorkshire Terriers, Irish Wolfhounds, Cairn Terriers, Maltese Terriers, Australian Cattle Dogs, Golden Retrievers, Old English Sheepdogs and Labrador Retrievers. Breeding studies in Maltese Terriers suggest liver shunts are actually inherited in this breed. Some genes potentially involved in the development of shunts have been identified. Inheritance of portosystemic shunts likely involves multiple genes and is complex, so figuring out how to prevent shunts through selective breeding is going to take some time and many research dollars. For now, pets diagnosed with shunts should not be bred for fear of perpetuating this condition.
The diagnostic evaluation of a pet suspected of having a liver shunt is extensive. First, your veterinarian has a suspicion for the presence of a shunt. This suspicion is often based on neurologic signs in a runty young dogor drooling after eating in a scrawny kitten. Specific blood tests to assess liver function are helpful. Diagnostic imaging, such as an X-ray with special dye, an ultrasound or a CT scan is necessary to pinpoint the exact location of the shunt and prepare for possible surgical closure of the shunting vessel.
Multiple methods have been developed to surgically correct liver shunts, but the goal of all the approaches is to close off the shunt and redirect the blood through the liver. Sending the blood through the liver allows the organ to resume multitasking. An “open” approach to shunt repair involves an abdominal incision in which the shunt is tied off or closed. A minimally invasive approach uses a catheter, which is guided by a special kind of live action X-ray machine, to insert a device that blocks the vessel. This is considered a “closed” approach because no incision is needed.
Sometimes, the shunt cannot be closed surgically, either because of the location of the vessel or because of high blood pressure in the liver. In those patients, diet and medications can help control clinical signs resulting from the liver shunt. Diets in patients with liver shunts restrict protein. Breakdown of protein during digestion releases compounds responsible for the neurologic signs related to the shunt, such as seizures and blindness. Restricting protein helps control these signs. In the gut, bacteria produce compounds that would normally be removed by the liver. When the compounds produced by the bacteria accumulate, they contribute to the clinical signs of a liver shunt. Decreasing their numbers improves the pet’s status. Veterinarians use two approaches to reduce bacteria: antibiotics and a medication called lactulose. Lactulose changes the internal intestinal environment, which decreases bacterial growth and also has a laxative effect, decreasing absorption of bacterial toxins by shortening the time stool is in the gut. Dogs treated with lactulose may need more frequent walks although some may simply just have soft stool.
While diagnostic testing, surgery or lifelong medication can seem like scary prospects when your favorite fur-person is in the midst of the discussion, keep in mind that many patients improve dramatically after treatment. With a highly skilled team of veterinarians and a dedicated pet family, the vast majority of pets with shunts amenable to closure recover and lead normal healthy and happy lives. Those who need to be managed medically can often enjoy a very good quality of life as well.